Study on the kinetics of formation process of emulsion of heavy oil and its functional group components

Enhanced oil recovery (EOR) by in situ formation of oil-in-water emulsion in heavy oil cold production technology has received growing interest from the petroleum industry. We present an experimental study of emulsification of model oils prepared by heavy oil and its functional group compositions dissolved into toluene brought into contact with a surfactant solution. The effects of functional group composition, emulsifier concentration, temperature, pH and stirring speed on the emulsification rate of heavy oil was investigated. A second-order kinetic model characterizing the temporal variation of conductivity during the emulsification has been established. The results show that acidic and amphoteric fractions exhibit higher interfacial activity, larger emulsification rate constant and faster emulsification rate. With the increase of emulsifier concentration, the emulsification rate constant increase to the maximum value at a concentration of 0.05 mol/L before decreasing. Temperature increase benefits the emulsification rate and the activation energy of the emulsification process is 40.28 kJ/mol. Higher pH and stirring speed indicate faster emulsification rate. The heterogeneity of emulsions limits the accuracy of dynamic characterization of the emulsification process and the determination method of emulsification rate has always been controversial. The conductivity method we proposed can effectively evaluates the emulsification kinetics. This paper provides theoretical guidance for an in-depth understanding of the mechanism and application of cold recovery technology for heavy oil.


Reagents and instruments
Dodecyl dimethyl betaine (BS-12, AR, Products of Shanghai Maclin Biochemical Technology Co., Ltd), Toluene (AR, Sinopharm Group Chemical Reagent Co., Ltd.), Crude oil was Gudao heavy oil (density 0.9282 g/mL, viscosity 965 mPa s at 50 °C), The experimental water was secondary distilled water.The crude oil underwent separation into functional group components through ion-exchange chromatography, with their compositions and properties shown in Table 1.
Table 1.The composition and properties of island heavy oil and its functional group components.

Separation of functional group components in heavy oil
The separation method of ion exchange chromatography is based on the functional groups of substances.Heavy oils can be divided into acidic, alkaline, amphoteric and neutral fractions.The process of separation of four components of heavy oil functional groups by ion exchange chromatography 58 is shown in Fig. 1.

Measurement of electrical conductivity
The heavy oil and its functional groups were separately dissolved into a toluene solution with a mass fraction of 0.05%.Dodecyl dimethyl betaine (BS-12) was prepared as a water solution with a concentration of 0.05 mol/L.The experimental device is shown in Fig. 2. The model oil phase and water phase were combined in the internal beaker in a volume ratio of 1:4 (oil to water).After temperature stabilization at 25 and a 30-min incubation period, the electric mixer and conductivity meter were activated, and the oil-water phase began to mix and emulsify to form an oil-in-water emulsion, with conductivity data measured every 20 s.

Observation of emulsion morphology
After the conductivity measurement, an appropriate amount of emulsion was taken immediately to prepare a microscope observation slide.The polarizing microscope was then employed to observe the microscopic morphology of the emulsion, and the corresponding images were recorded.

Establishment of emulsification kinetics equation
Figure 3 illustrates the schematic changes in water phase conductivity during the emulsification process.Due to the mixing of the oil phase, the conductivity of the water phase gradually decreases.Once a uniform and stable emulsion is formed, the conductivity will remain dynamically constant.By substituting the experimental data into the kinetic equations of different reaction order, it is found that the characteristics of second-order reaction kinetics are consistent, and the kinetic equations are established accordingly: When t = 0, Δκ t = 0. the integral of formula (1) is: It is deduced that: (1) (2) (3) Figure 1.Flow chart of four-component separation of heavy oil functional groups.
It can be seen from Eq. ( 3) that the t/Δκ t of the second-order emulsification reaction has a linear relationship with t.According to the change in water conductivity at different time intervals, the relationship curve of t/Δκ t against t was drawn for linear fitting.The emulsification rate constant k was determined according to the slope and intercept.
In the formula: Δκ is the maximum electrical conductivity change, μs cm −1 ; Δκ t is the change of conductivity at time t, μs•cm -1 ; k is the emulsification rate constant, μs•(cm•s) -1 ; t is the emulsification time, s; κ 0 is the value of initial conductivity, μs•cm -1 ; κ t is the value of conductivity at time t, μs•cm -1 ; κ e is the value of electrical conductivity at emulsification equilibrium, μs•cm −1 .www.nature.com/scientificreports/

Effect of heavy oil components on emulsification rate
Figure 4 shows the change curve of water phase conductivity over time during emulsification of different active components of heavy oil.With the high-speed mixing of the two phases of oil and water, the emulsion is formed.The electrical conductivity of the crude oil emulsion depends on its water content and the dispersion degree of the water particles 59 .With the emulsification process, the oil phase is dispersed in the water phase, and the conductivity of the water phase decreases.When the emulsion is stable, the water phase conductivity will remain dynamically stable.The rate of conductivity change can quantitatively characterize the emulsification rate.Equation (3) was employed to fit the curves of conductivity changes over time during the emulsification of different model oil and water phases.The fitting results (Fig. 5 and Table 2) showed that the correlation coefficient R 2 for each fitting curve under the experimental conditions was greater than 0.994.This indicates a strong fit between the experimental results and the assumed emulsification equation.The simulated emulsification process of the oil phase and water phase conforms to the second-order reaction equation.The order of the emulsification rate constant k in the simulated oil-phase emulsification process of different heavy oil components is as follows: acid fraction > amphoteric fraction > heavy oil fraction > alkaline fraction > neutral fraction.Acidic and amphoteric fractions can form a stable interfacial film at the oil-water interface 60   indicating a certain error between the two.Because of the highly conditional reaction emulsification, it is difficult to achieve an ideal emulsification state.The Δκ measured in experiments is lower than the fitting value, with a maximum error of 9.31% (for the alkaline fraction), which is considered acceptable.
Since droplet particle size has a significant effect on the agglomeration stratification of emulsion, the particle size of the dispersed phase of emulsion serves as a crucial index for the evaluation of the formation results of an emulsion.The larger the droplet size, the greater the probability of collision and polymerization, which is detrimental to the formation of stable emulsions 61 .Therefore, optimal emulsification conditions should facilitate the generation of small and uniform droplets.It can be seen from the microscopic images of emulsions with different functional groups of heavy oil in Fig. 6 that emulsions formed by acidic and amphoteric fractions exhibit smaller and more uniform droplet size, coupled with faster emulsification rates.Conversely, emulsions formed by the neutral fraction display large, uneven droplets with a tendency towards fusion and aggregation.This results in a less stable emulsion and unfavorable emulsification results.

Effect of emulsifier concentration on emulsification rate
Equation (3) was employed to fit the water phase conductivity of emulsifiers at different concentrations.The results are shown in Table 3 and Fig. 7.The analysis indicates that the correlation coefficients of the fitting curves are greater than 0.991, affirming the suitability of this fitting equation for the data (Fig. 8).With an increase in emulsifier concentration, the emulsifying rate constant the emulsifying rate constant first rose and reached its maximum value at a concentration of 0.05 mol/L before decreasing.When the emulsifier concentration is below 0.05 mol/L, its increase causes higher adsorption of emulsifier molecules at the oil-water interface, facilitating the mixing of oil and water phases and the emulsification process.However, at a concentration of 0.05 mol/L, the emulsification rate constant reaches a maximum due to the saturation in interfacial facial mask adsorption 62,63 .Further increase in the emulsifier concentration after 0.05 mol/L results in a decrease in the emulsification rate constant and a slowdown in the emulsification rate.
From the microscopic images of model oil emulsions under different emulsifier concentrations in Fig. 9, it can be seen that as the emulsifier concentration increases, the particle size of the emulsion droplets decreases, while the number of droplets increases.However, there is a point beyond which the particle size is difficult to further reduce.When the emulsifier concentration increases after 0.05 mol/L, it tends to flocculate into vesicles or micelles 64 .This will increase the solubility of excess emulsifier molecules, which is not conducive to their adsorption at the interface, hinder the emulsification process, and reduce the rate constant.

Effect of temperature on emulsification rate
The change curve of water phase conductivity with time in the emulsification process of model oil at different temperatures is shown in Fig. 10.The data of this group are fitted to Eq. ( 3), and the fitting results are shown in Fig. 11 and Table 4.All the fitting curves exhibited correlation coefficients greater than 0.997 at different temperatures.This suggests that the emulsification process conforms to the assumed Eq. ( 3) and follows a second-order rate reaction.As can be seen from Table 4, with the increase in temperature, the emulsification rate constant also rises.Notably, this increase is more greater at higher temperatures.The colloidal stable system with asphaltene as the core in model oil is no longer stable at high temperature and shows signs of dispersion 65 .The higher the temperature is, the more dispersed the micelles will eventually disintegrate, resulting in lower viscosity of crude oil, increased rheology, and easier mixing with water.Under high temperatures, the solubility of the active components (acidic and amphoteric fractions) in petroleum increases 66 , which will change the strength and toughness of the oil-water interface film and accelerate the emulsification process.According to the Arrhenius It can be seen from the microscopic images of model oil emulsions at different temperatures in Fig. 12 that as the emulsification temperature increases, the droplets of model oil emulsions become smaller, exhibiting more uniform particle size, and a narrower particle size distribution.Higher temperature proves beneficial to the emulsion formation, with the emulsification rate increasing alongside.When emulsified for the same time, the smaller the average particle size of the formed emulsion, the greater the propensity for a uniform and stable model oil emulsion to form 67 .

Effect of pH of water phase on emulsification rate
Alkali flooding is a common method to enhance tertiary oil recovery.The production of heavy oil is increased by in-situ emulsification of surfactant generated by the reaction of injected alkaline solution with natural organic acid in heavy oil 64 .Therefore, the alkalinity of the water phase bears a significant correlation with the formation of heavy oil emulsions.The curve (Fig. 13) representing the conductivity of the water phase over time under different pH conditions was fitted to Eq. ( 3).The results (Fig. 14 and Table 5) showed that the correlation coefficients of the experimental fitting curves were all greater than 0.997.This indicates that the fitting results are highly satisfactory.The emulsification rate constant exhibited an increase with the rise in the pH in the water phase.Given that heavy oil contains many acidic functional groups 28 , they serve as the key factors in emulsion formation.When the pH is greater than 7, the acidic groups in the heavy oil can be activated by alkali, leading to a saponification reaction for the formation of a surfactant that exerts a certain emulsification effect 68 , thereby accelerating emulsion formation.
From the microscopic images of model oil emulsions formed under different pH conditions in Fig. 15, it can be seen that the average particle size of the emulsion formed at pH = 7 is much larger than that at pH = 10, 11, and 12.At pH = 7, the emulsion mainly depends on the energy of stirring.The effect of surfactant is minimal, with a slower emulsification rate and larger particle size of the emulsion formed within the same duration.At pH levels of 10, 11, and 12, the surfactant formed by the reaction of the acidic component of the model oil with the alkali exhibits a favorable emulsification effect 69 .This substance synergizes with the external stirring to accelerate emulsification and yield smaller emulsion droplets.It becomes clear that the greater the pH of the water phase, the better the emulsification effect.www.nature.com/scientificreports/

Effect of stirring speed on emulsification rate
Figure 16 shows the variation curve of conductivity over time during emulsification at different stirring speeds.
In general, the emulsification process does not occur spontaneously.The mixing and stirring of the oil phase and water phase (i.e., providing mixing energy) is one of the necessary conditions for emulsion formation 70 .In the stirring process, the oil phase undergoes deformation and elongation and breaks into smaller oil droplets that ultimately coalesce to form an emulsion 71 .The change curves of emulsion conductivity over time at different stirring speeds were fitted to Eq. ( 3).The fitting results (Fig. 17, Table 6) indicate that the correlation coefficients of each fitting curve were greater than 0.997, demonstrating that the experimental data of each group were well-fitted to Eq. ( 3).When the stirring speed increased from 210 r/min to 240 r/min, the emulsification rate constant also saw a significant rise from 5.32 × 10 -6 μs/(cm s) to 1.34 × 10 -4 μs/(cm s), revealing the strong influence of different stirring speeds on the emulsification rate constant.In this experiment, stirring serves as the only external energy for mixing the oil phase and water phase.Different stirring speeds provide different mixing energies, which determines the rate of the emulsification process.The high-speed stirring causes the oil droplets to deform and stretch, and then shear and break into small particles.The increase of stirring speed and time will make the oil droplets of emulsion smaller.And the smaller the particle size of the oil droplets is, the easier it is to get a evenly   mixed oil-in-water emulsion.The relative error between the fitted Δκ value and the experimental value is the largest when the stirring speed is 210 r/min.At this time, the mixing energy provided by stirring is at its lowest.As a result, the relative emulsification effect is the worst, along with the smallest emulsification rate constant.
The microscopic images of model oil emulsion at different rotational speeds in Fig. 18 provide further confirmation that the stirring speed is the most important factor affecting the emulsification effect.As the stirring speed increases from 210 to 240 r/min, there is a significant variation in emulsification results.At a stirring speed  of 210 r/min, the emulsion exhibits a large average particle size and the droplets have irregular shapes, showing a tendency to coalesce with each other.In contrast, at 240 r/min, the obtained emulsion features a smaller average particle size, uniform droplet sizes, and regular shapes.The emulsification result at high speed is superior at the same time.

Conclusion
(1)(1) The kinetic model of heavy oil emulsification process was established for the first time by using the conductivity method.It was proposed that the change of water phase conductivity during emulsification conforms to the second-order rate equation: The emulsification data under different experimental conditions were well-fitted, with linear correlation coefficients all greater than 0.991, indicating that the change in water phase conductivity in the emulsification process conformed to the second-order reaction equation.The conductivity method has the advantages of simple, non-destructive, low cost, easy on-line implementation and wide application, which provides theoretical guidance for understanding the mechanism and application of heavy oil cold production technology.
(2)(2) Acidic and amphoteric fractions can form a stable interfacial film at the oil-water interface, this prevents coalescence between droplets, and facilitates the formation and stability of the emulsion, resulting in a faster emulsification rate during the stirring process.With an increase in emulsifier concentration, the emulsifying rate constant the emulsifying rate constant first rose and reached its maximum value at a concentration of 0.05 mol/L before decreasing.The emulsification rate constant continues to increase with rising temperatures.The higher the temperature, the greater the increase in emulsification rate constant.The activation energy for the heavy oil emulsification reaction is calculated to be E a = 40.282kJ/mol.In the emulsification process of the oil phase and water phase, the stronger the alkalinity of the water phase, the larger the emulsification rate constant and the emulsification rate.Different stirring speed strongly affects the emulsification rate constant, with the faster stirring speed leading to larger emulsification rate.

Figure 2 .
Figure 2. Diagram of the experimental setup for conductivity determination in an emulsification process.

Figure 3 .
Figure 3. Schematic diagram of the change of aqueous conductivity during emulsification.

Figure 5 .
Figure 5. Kinetic fitting curves of emulsification processes of functional components of different heavy oils.

Figure 7 .
Figure 7.The change of conductivity over time during model oil emulsification was simulated at different emulsifier concentrations.

Figure 8 .
Figure 8. Kinetic fitting curves of emulsification processes with different emulsifier concentrations.

Figure 9 .Figure 10 .
Figure 9. Microscopic image of model oil emulsion at different emulsifier concentrations.

Figure 11 .
Figure 11.Kinetic fitting curves of emulsification processes at different temperatures.

Figure 15 .
Figure 15.Microscopic images of model oil emulsions at different pH.

Figure 16 .Figure 17 .
Figure 16.The change of conductivity over time during the emulsification process of model oil is simulated at different speeds.

Figure 18 .
Figure 18.Microscopic image of model oil emulsion at different rotational speeds.
, inhibiting coalescences between droplets.This supports the formation and stability of emulsions to induce a faster, emulsification rate during the stirring process.The kinetic fitting values of Δκ for each model oil component are greater than the experimental values, Figure 4. Changes in conductivity over time during emulsification of functional group components of different heavy oils.

Table 2 .
Kinetic fitting results of emulsification process of different heavy oil functional group components.

Table 3 .
Kinetic fitting results of emulsification process with different emulsifier concentrations.

Table 4 .
Kinetic fitting results of emulsification processes at different temperatures.

Table 5 .
Kinetic fitting results of emulsification process at different pH.

Table 6 .
Kinetic fitting results of emulsification process at different speeds.